1. 12. Mass Wasting
Classifications (Definitions, processes and controlling factors)
Examples (Appling knowledge of processes)
Prevention of Mass Wasting (limiting and eliminating)
I will give some more detail for the objective of the chapter
I indicate the questions to focus on

2. Terms and Concepts to Learn
Definitions
Flow, Slide, Fall
Controlling Factors
Gravity
Water
Shear strength
Examples
Mt. St. Helens
Yosemite Rock fall/slide
Gros Ventre Slide
St. Francis Dam, Calif.
Be able to distinguish a flow from a slide.
How do controlling factors increase or reduce tendency of mass wasting?
How did the controlling factors lead to these events?
How would you use your knowledge of the controlling factors to reduce risk?

3. Classification of Mass Wasting

4. Classification of Mass Wasting
 Material
Debris
Saturated Debris
Debris or Rock
Bedrock
 Velocity
Imperceptibly Slow
Slope and Material Dependent <5 km/hr
Very Fast 100 km/hr
Slow-mod. (short)
Fast
Classification
Creep
Earth Flow
Mudflow
Avalanche
Rotational Slide
Rock Slide
Debris Fall
Rockfall
Flow
Type of Movement
Slide
Fall

5. Creep Imperceptibly slow flow Expansion - contraction
Heating – Cooling
Freeze – Thaw
Fig. 9.6

6. Earth Flow and Rotational Slide
Debris (soil) both slides and flows
Rotational Sliding
Zone of Depletion
Surface of Rupture
Flow
Zone of Accumulation
Surface of Separation

7. Slump/Earth- Flow Zones of Depletion Zone of Accumulation Crown
Left Flank
Zones of Depletion
Crown
Main/Minor Scarps
Head Transverse Cracks
Longitudinal Fault Zone
Zone of Accumulation
Transverse Ridges and cracks
Radial cracks
Foot/Tip/Toe
Right Flank
Head
Foot
Kehew, Fig

8. Rock Slide and Fall Bedrock may slide and/or fall
Weathering reduces bedrock strength
Chemical
Mechanical
Eventually gravity wins
Earthquake triggered

9. Talus Slopes The result of Rock Avalanches Mechanical weathering
Rock falls and slides
Crushing and abrasion (more mechanical weathering)
Rock Avalanches
Slopes of rock fragments may let go and careen downhill as a very fast flow

10. E.g. Unavoidable Rock Fall
The Old Man of the Mountain, Cannon Mtn. NH

11. Mt. St. Helens Landslide triggers eruption Eruption causes Mudflows
Reduced shear strength from earthquakes and bulging
Increased shear force as bulge grows and slopes steepen
Eruption causes Mudflows

13. Gros Vantre Slide Sandstone and debris on Impermeable shale
Saturation of sandstone and lubrication of shale
Both reduced shear strength (added to shear force)
Shear force overcomes shear strength
Sandstone and debris slide

14. Shear Force vs. Shear Strength
Driving Forces
i.e., Shear Force
Fs = W sin q
Fs: Shear force
W: Weight
q : Dip of slope

15. Shear Force vs. Shear Strength
Resisting Forces
Friction and Cohesion of Soil or Rock
Ff = m (W cos q)
m: coefficient of static friction

16. Shear Force vs. Shear Strength
Resisting Forces
i.e., Shear Strength
Friction and Cohesion of Soil or Rock
S = C + se tan f
S: Strength
C: Cohesion
se: Effective Stress
f : Angle of internal friction

17. Safety Factor S resisting forces S Driving Forces
If S.F.>1 then failure occurs
S.F.=

18. Effective Stress and StrengthP
P+DP
Before Rainfall After Rainfall
se = sT – P se = sT – (P+DP)
sT: Total Stress
P: Fluid pressure of ground water (or soil water)
se: Effective stress (stress supported by the soil skeleton)
Note: fluid pressure is negative (less than atmospheric) if unsaturated and becomes positive when saturated

19. Stability Analysis for Rotational Slides
S.F. = SLR / WX
S: Shear Strength
L: Langth of potential surface of rupture
R: Moment Arm
W: Weight of Block
X: Length of gravity moment arm X R
C.G. S L

20. Use Knowledge of Mass Wasting to Avoid Risks
Be able to recognize geologically unstable situations

21. Understanding Mass Wasting
Development causes:
Increased shear force
Steepened slope
Added weight
Decreased shear strength
Devegetation
Reworking of fill
Saturation of soil

22. Reduce Risks Some solutions include: Increase shear strength
Re-compact soils
Re-vegetate soil slopes
Construct retaining wall with anchors
Prevent Saturation
Prohibit over-irrigation
Install surface drains
Install subsurface drains

23. Reduce Risks Increase shear strength with iron rods and anchors
Remove risk